Method of increasing metal temperatures in the cold end of air preheaters



June 27, 1961 H. J. BLASKOWSKI 2,990,161

METHOD OF INCREASING METAL TEMPERATURES IN THE COLD END OF AIR PREHEATERS Filed. Nov. 29, 1954 o GAS TUBE WALL O l 01 0 0| I O 1 A AIR FILM P o; 7

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SCREEN INVENTOR HENRY J. BLASKOWSKI ATTORNEY nite The present invention relates to an improved method of increasing the metal temperature at the cold end air preheaters of the type employed in boiler operation wherein air is heated by being passed in indirect heat exchange relation with combustion gases leaving a boiler. The invention has specific relation to such a method embodying the use of a fluidized discrete material.

In the operation of air heaters of this type one of the ever present problems has been corrosion of the metal at the cold end of the air heater due to condensation from the combustion gases forming on this metal as a result of its being cooled below the dew point of these gases with this condition being prevalent when the boiler with which the air heater is associated is operated at low loads or even at high loads when the humidity of the combustion supporting air is high. Because of the make up of these gases this condensation takes the form of sulfuric acid which quickly deteriorates the metal of the air preheater. Various methods and means have been devised to prevent this condensation from forming and thus prevent the resulting corrosion such as recirculating a portion of the air or otherwise initially heating the air entering the air preheater. It has also been the practice to construct the cold ends of these air heaters as detachable units which can be replaced when necessary without replacing the entire air heater.

The present invention contemplates an improved method for raising the metal temperature at the cold end of such air preheaters in order to lessen the likelihood of condensation taking place when the boiler with which such an air heater is associated is operated at low loads, or when the humidity of the combustion supporting air is high. In accordance with the invention a discrete ma terial is retained in contact with the surface of the material of the air preheated at the cold end of the air preheater and which surface is contacted by the hot combustion gases. These gases pass through this discrete material and maintain it in a fluidized condition thereby greatly increasing the rate of heat transfer between the combustion gases and the material resulting in raising the temperature of the material and accordingly lessening the possibility of the metal temperature being lowered to or below the dew point of the combustion gases.

This result is brought about because of the inherent characteristic properties of a fluidized bed or medium, one of which is that there is a very high rate of heat transfer between the discrete material of the bed and the fluidizing gas and between this material and a heat exchange surface that is in contact with the material, this latter being brought about due to the fact that the fluidized medium breaks down the insulating gas layer immediately adjacent the surface. A discrete material of predetermined particle size will become fluidized when a stream of gas at or above a predetermined velocity is passed through the material and when in this fluidized state the particles of the material will move rapidly and the entire mass of material will be in an agitated state resembling a boiling liquid with the violence of the agitation depending upon the velocity of the gas passing through the material. When in this fluidized state the material is not carried along with the fluidizing gas and although the body of material will be expanded so that ice it occupies considerably more volume than when in the non-fluidized state the fluidizing gas passes through the body of material and leaves the same at what is termed a disengaging zone. This type of fluidization is sometimes referred to as dense phase fluidization and when the term fluidization is used through this application reference is had to this type of fluidization. With a given particle size the velocity of the gases passing through the material must be maintained within a minimum and maximum limit in order to have fluidization. The minimum limit is that minimum velocity which is necessary to fluidize the material and below which the material remains packed and in a non-fluidized condition while the maximum velocity is that above which the material or a substantial portion of the material will be carried along with the combustion gases whereupon the particles will be literally blown away.

It is an object of this invention to provide an improved method for raising the temperature of the material at the cold end of a combustion gas to air air preheater to reduce the likelihood of corrosion at the cold end.

Other and further objects will become apparent to those skilled in the art as the description proceeds.

For clarity and ease of explanation reference will be made to the accompanying drawing wherein there is disclosed a diagrammatic representation of an organization for carrying out the novel method of this invention and in which representation:

FIGURE 1 represents a tubular air heater with a side Wall of the air heater being removed in this showing to disclose the internal arrangement of the air heater.

FIGURE 2 is a sectional view taken along line 22 of FIG. 1.

FIGURE 3 is a sectional view through a portion of one of the tubes with the portion being located in the cold end of the air heater and the section taken along line 33 of FIG. 1.

FIGURE 4 is a view similar to that of FIG. 3 but showing a modified construction.

FIGURE 5 is a diagrammatic representation of the temperature gradient being that achieved with the improved method of this invention.

FIGURE 6 is a diagrammatic representation similar to that of FIG. 5 but showing the resulting temperature gradient that is obtained prior to the improved method of this invention.

Referring now to the drawing, wherein like reference characters are used throughout to designate like elements, the air heater shown therein comprises a tube bundle generally designated 10 and composed of nu merous parallel laterally spaced tubes 12 which are connected at their opposite ends to tube sheets 14 and 16 respectively, with the bundle being enclosed within the duct or gas pass 18 through which combustion gases are upwardly directed as indicated by arrows 20.

Air is directed laterally over the tubes 12 and in order to have the air pass across the tubes several times the bafile plates 22, 24 and 26 are provided as shown di viding the interior of the air heater into the latter passages 28, 30, 32, and 34 with the cold air entering passage 28 from inlet duct 36 and serially passing through these respective passageways in the manner indicated by arrows 38 with the hot air being discharged into duct 40 which leads to the desired point of use.

The cold end of the air heater which is the portion of the air heater wherein the condensation and resulting corrosion occurs as mentioned herein'before is the end at which the cold air is first introduced into the air heater with this end being the portion of the air heater included in the passage 28 in the illustrative organization shown and it would be the portions oftubes 12 which extend through this passage within which condensation of the combustion gases would be likely.

In order to raise the temperature of the tube metal in these tube portions extending through passage 28a discrete material is positioned within these tube portions so as to be in contact with the inner wall surface thereof. This material may be retained within these tube portions 'by means of a pair of longitudinally spaced screens 42 and 44 (FIG. 3) with screens 42 secured at the upper end of the tubes and screen 44 secured within the tube at the beginning of the portions extending through passage 28. These screens have a mesh which is suificiently small to prevent the particles of the discrete material from passing therethrough. In lieu of the foregoing arrangement for retaining the material within these tube portions the modified arrangement shown in FIG. 4 may be employed with this arrangement consisting of tubular screen 43 spaced inwardly from the inner wall of the tube portions and secured at its ends to this inner surface of the tube thereby forming an annular chamber with the tube wall within which the discrete material is retained with this screen of course also having a mesh sulficiently small to prevent the passage of the discrete material therethrough.

In operation, the combustion gases passing upwardly through the tube 12 pass through the screens which retain the discrete material within the tube portions extending through passage 28 thereby fluidizing this discrete material resulting in greatly increasing the heat transfer from the heating combustion gases to the tube wall causing the temperature of this wall to be substantially increased. The particle size of the material is chosen so that with the maximum gas velocity anticipated through duct 18 and with the gas flowing through all of the tubes 12 the material will be fluidized and the velocity through the tube will not be above that which will produce fiuidization of the particular material. As mentioned hereinbefore the velocity for a particular particle size must be within a predetermined minimum and maximum in order to result in fiuidization of the material. Thus with a particle size chosen for maximum gas flow through duct 18 fluidization will occur only when the gas velocity through conduits 12 remains within the maximum and minimum velocity limits for the size of the particles.

In order to increase the minimum velocity through duct 18 which will result in fluidization of the material within the conduit portions extending through passage 28 means are provided to control the gas =fiow through certain groups of the tubes, With this means being diagrammatically represented for the purpose of explanation as dampers 48 provided at the inlets of the several rows of tubes 12 with each of these dampers pivoting about its axis 50 and being individually operated between a position where they completely obstruct gas flow through the particular row of tubes with which they are associated and a position where they are entirely free of any restrictive action. In this manner, as the gas flow through duct 18 approaches the lower limit required for fluidization when the gases are passing through all the tubes 12 one of the rows of tubes may be shut otf with respect to gas flow therethrough correspondingly increasing the gas flow through the remaining tubes with this procedure being followed until the practical minimum number of tubes 12 are open to gas flow thereby greatly increasing the range of gas flow through duct 18 over which the present invention may be practiced. It should be noted that unless the material within the tube portions extending through passage 28 is fluidized no substantial increase in the metal temperature of the tube portions will result and in fact a decrease in temperature is likely.

being heated with the novel method of the present invention while FIG. 6 shows how the temperature drops with the conventional method of heating air with combustion gases.

In the BIG. 5 diagram, A represents the temperature of the combustion gases, B represents the temperature at the inner surface of the tube wall, C represents the temperature at the outer surface of the tube wall and, D represents the air temperature at the inner side of the insulating air film which surrounds the outer surface of the tubes. In the illustration of FIG. 6 the temperatures A, B, C, and D correspond respectively tothe temperatures of A, B, C, and D in the illustration of FIG. 5 except that in the illustration of FIG. 6 the temperatures are those prevailing with the conventional method of heating. It will be noted that the temperature drop from A to B in the FIG. 5 illustration is very much less than that in the FIG. 6 illustration. This is because with the fluidized medium the insulating gas film is for all practical purposes destroyed and the rate of heat exchange between the combustion gas and the tube wall is therefore very much increased. Because the temperature drop between A and B is much less with the method of this invention the metal temperature of the tube is correspondingly much higher and while the temperature drop between B and C, and B and C is substantially the same as shown in the graphic illustrations of FIGS. 5 and 6 the tube metal temperature in the illustration of FIG. 5 is very much greater and accordingly the likelihood of condensing the combustion gases on the surface of the tubes is much less. As an example, if the gas and the air temperatures, i.e., the temperature at A and A and the temperature at D and D in the illustration of FIGS. 5 and 6, were 280 F. and F., respectively, the temperature at B may be approximately 200 F. and at C 180 F. and while the temperature at B would be about 270 F. and at C would be about 250 F. for the same mass flow of air and gas in each instance. Thus the average metal temperature with the novel method of this invention would be about 70 F. higher than that with the method heretofore practiced thereby greatly decreasing the likelihood of condensation and deterioration at the cold end of the air heater.

While the invention has been described in detail it is to be understood that it is not to be limited except in accordance with the scope of the appended claim.

What I claim is:

-In a combustion gas to air air heater of the indirect heat exchange type wherein combustion gas that is flowing through a duct is directed in parallel through a plu rality of passages formed by metal wall members and contacts one side of said metal wall members and air to be heated is passed in contact with the other side of said wall members the method of increasing the metal temperature at the cold end of the air heater to lessen the likelihood of condensation thereat comprising retaining a layer of particulate solid material solely in engagement with the combustion gas contacted surface at said cold end while maintaining a substantial portion of the transverse section of said passages at said cold end free of said material and while maintaining normal contact of the air with the contacted surface at the cold end and passing the combustion gas through said material at a velocity such that said material is fluidized.

References Cited in the file of this patent UNITED STATES PATENTS 2,389,850 Gunter Nov. 27, 1945 2,550,722 Rollman May 1, 1951 2,690,051 Peskin Sept. 28, 1954 OTHER REFERENCES Heat Transfer Characteristics of Fluidized Beds, pages 1135 through 1147 of Industrial and Engineering Chemistry for June 1949, vol. 41, No. 6. 

